Logic bricks for scene objects
Logic bricks are attached to a single object. You can use message actuators and sensors to get an object to respond to another objects sensor.

Using a python controller it is possible to access other objects in the scene.

# Loop through all other objects in the scene
sce = bge.logic.getCurrentScene()
print("Scene Objects:", sce.name)
for ob in sce.objects:
    print("   ", ob.name, ob.worldPosition)